Third generation partnership project (3GPP) mobile communication systems based on a wideband code division multiple access (WCDMA) radio access technique are widely spread all over the world. High speed downlink packet access (HSDPA) that can be defined as a first evolutionary stage of WCDMA provides 3GPP with a wireless access technique that is highly competitive in the mid-term future. However, since requirements and expectations of users and service providers are continuously increased and developments of competing radio access techniques are continuously in progress, new technical evolutions in 3GPP are required to secure competitiveness in the future.
One of the systems being taken into consideration after the third generation is an orthogonal frequency division multiplexing (OFDM) system that can reduce the intersymbol interference effect with low complexity. The OFDM transforms serially inputted data symbols into N parallel data symbols and transmits the parallel data symbols with loaded on N sub-carriers separated from each other. The sub-carriers maintain orthogonality in terms of frequency. Each of orthogonal channels experiences mutually independent frequency selective fading, and the spaces between transmitted symbols become wider, and thus interference between the symbols can be minimized. Orthogonal frequency division multiple access (OFDMA) is a multiple access method that realizes a multiple-access by independently providing some of available sub-carriers to each user in a system that uses OFDM as a modulation method. The OFDMA provides frequency resources, which are referred to as sub-carriers, to each user, and each of the frequency resources are independently provided to a plurality of users, and thus the frequency resources are generally not overlapped with each other. As a result, the frequency resources are mutual-exclusively allocated to each user.
One of the major problems of the OFDM/OFDMA is that peak amplitude of a transmission signal can be considerably higher than average amplitude. This peak-to-average power ratio (PAPR) problem is originated from the fact that an OFDM signal is the sum of N sinusoidal signals on sub-carriers different from each other. In order to save transmission power, it is needed to lower the PAPR.
One of the systems proposed to lower the PAPR is single carrier-frequency division multiple access (SC-FDMA). SC-FDMA is a type that combines frequency division multiple access (FDMA) with existing single carrier-frequency division equalization (SC-FDE) method. The SC-FDMA has a characteristic similar to that of the OFDMA in that signals are modulated and demodulated in a time domain and a frequency domain using discrete Fourier transform (DFT), but it is advantageous in saving transmission power since the PAPR of a transmission signal is low. Particularly, in connection with usage of a battery, it is advantageous for an uplink that connects a user equipment sensitive to transmission power to a base station.
In order to efficiently restore data at a receiver, channel information should be obtained. The channel information is used for modulating and demodulating the data or scheduling users. Generally, the channel information is obtained based on a pilot contained in a signal transmitted by a transmitter. However, an efficient pilot structure has been not widely known yet.